A few people have asked about keeping heating on a thermostat versus putting it on a few hours in the morning and a few in the evening. When I was at uni one of my physics professors assured us impoverished students that it was better to leave it on.
I turn the boiler on to 24/7 setting and then allow my programmable thermostat to maintain 16 in the day and 14 at night.
The heat transfer (loss) from warm inside to the cold outside is proportional to the temperature gradient between the two (ie. the temp diff between inside and out), so cold weather requires more energy to maintain the constant inside temperature than a warmer day would.
It isn't a straight calculation though because it is complicated by the wind chill factor. So more heat will be lost from a house exposed to strong winds than the same house on a non-windy but colder day.
Also, if you don't have cavity walls and one side of your house is in the driving rain it can make the damp and this will reduce its insulation value.
All the materials in your home, the bricks, the plasterboard etc, have a specific heat value. This is a measure of the quantity of heat a material can hold. Most materials don't hold much heat, though others like stone and water hold quite a lot.
The advantage of leaving the heating on a low set thermostat 24/7 is that the fabric of the house warms up and acts as a heat sink. This reduces the fluctuations in internal temperature between clear days and very cold nights.
If you were to turn off the heating in the day, say whilst out at work, and the fabric of the building cooled down when you heat it back up again you will need more energy to get the house back to a comfortable temperature, as before the inside can feel warm, the fabric of the house will 'soak' up some of the heat generated.
I hope that makes some sense.
So let it get a little colder at night but don't let the fabric of the house get cold. Once the house gets cold its very expensive to get it warm again.